1. Field of the Invention
The present invention relates generally to the field of audio conferencing and audio conferencing systems, and more particularly to digital linking of multiple auto-mixer systems.
2. Background of the Invention
In audio conferencing systems where more than one microphone is used, auto-mixing helps to enhance overall system sound quality by selectively turning on (opening) the microphones that contains strong signal activities (e.g., speech, music, etc.) and turning off (closing) those microphones that contain relatively insignificant signal activities (e.g., background noise, etc.). Auto-mixers are used to enable sound capture devices such as a microphone. Auto-mixers can selectively enable and disable microphones in an audio conference and can modify attenuation of audio signals. Conventional audio conferencing systems often implement auto-mixing in systems with numerous devices, thus creating overly complex and burdensome audio conferencing systems. In conventional systems, there is significant difficulty coordinating multiple devices with multiple auto-mixers. Analog mixers support linking, but fail to support additional features or options beyond merely linking signals. Conversely, conventional digital auto mixers do not perform as well or do not provide linking capabilities.
Conventionally, an auto-mixer is designed to improve the speech of active participants in an audio conference, rendering the audio-conferenced sounds to be more intelligible (due to, e.g., less comb filtering effects, lower ambient noise level, less reverberant effects, etc.), but also helps to prevent the conferencing system from howling.
In large auto-mixer systems, microphone input channels may be distributed among many devices. Rather than transmitting these signals individually to a central processing device for auto-mixing, greater efficiency is gained by distributing audio signal processing functions of an auto-mixer among multiple devices in an audio conferencing system. Subsequently, a smaller number of gated mixes of microphone signals may be transmitted between devices.
With regard to noise levels, noise thresholds are used to determine whether a microphone is experiencing signal activity. In general, noise threshold determination is based on an adaptive noise floor measurement (automatically or manually adjusting threshold). In many implementations, the speech (or other sound) should be a certain number of dB louder than the noise floor. This dB difference is sometimes called the “adaptive threshold,” which may be adjustable by the user.
One approach to circumvent the acoustic feedback problems associated with gating multiple microphones is to use a smaller system gain as more microphones are added to the system. In other words, by reducing the system gain, for example, number of microphones (NOM) count can be increased to permit more microphones to be gated open before feedback occurs. Experimental studies have shown that for each doubling of the number of microphones added to the system, the system gain should, in many cases, be lowered by 3 dB to avoid feedback. Limiting the maximum NOM count may also help to prevent feedback.
In some cases, noise in an area with microphones may have widely-varying signal amplitudes and can create a false indication at the auto-mixer that noise activity exists near one or more microphones. Actual average noise levels, however, may be at low levels. In an environment that is otherwise relatively devoid of other sounds or noise activity, an intermittent noise fluctuation may generate sufficient sound signal activity so as to exceed the noise floor measurement, resulting in a microphone turning on. Thus, the fluctuating noise may be sufficient to trigger an auto-mixer and gate a microphone into an open state. Even though the fluctuating noise is at a low level, the peak noise level may be high enough above the average noise level to gate one or more proximate microphones into an open state.
As also mentioned herein, some conventional auto-mixing systems are capable of linking. Linking enables multiple auto-mixers to share signals, information, and parameter data for the purpose of mixing signals from assigned microphones. Linking of multiple auto-mixers enables a mixed signal output. For example, where two auto-mixers exist, each auto-mixer may have eight microphones assigned and sound signal activity may be picked up by microphones assigned to the auto-mixers. Although there are two auto-mixers in the given example, by linking them, sound signals picked up from the sixteen assigned microphones can be shared and mixed to produce an arbitrary number of sound signal outputs. Thus, in turn, can be used to increase the quality of the sounds in, for example, an audio conferencing scenario. However, conventional systems are problematic and cannot perform gating logic functions, option control logic, and arbitration between local and remote microphones. Conventional linking is accomplished using analog signals which are not capable of performing gating logic functions.
In conventional audio mixing systems, limitations exist in a single complex and expensive auto-mixer (where automatic mixing and linking is irrelevant). In the context of conventional digital linking systems, seamless linking capabilities do not exist. Most of the prior art concerning auto-mixers are implemented in analog circuitry. While many of these auto-mixers are capable of linking, the link is accomplished with analog signals which are rather simple and not capable of many desired functions. Digital implementations are either designed in a single large device (where auto-mixer linking is not needed), or do not have the seamless linking capabilities that are desired.
Therefore, there is a need for a system for digital linking of multiple microphones where features and functionality of the multiple microphone systems are linkable.